The evolution of efficient scalable solvers and multi-physics codes in the past decade has considerably increased the requirements on computational needs for high-fidelity simulations of nuclear reactors through resolution of more heterogeneity in the physical models. In this paper, we present the Reactor Geometry (and mesh) Generator (RGG) toolkit, which is a part of MeshKit library developed at Argonne National Laboratory (ANL). The extensions of the RGG toolkit have been used to create PWR, ABTR, VHTR, MONJU and several other types of reactor geometry and mesh models for consumption in state-of-art physics solvers. RGG uses a lattice hierarchy-based approach to create these reactor core models and has been designed to scale on even large models with up to 1 billion hexahedral elements. Details on the RGG methodology and description of the parallel aspects of the tool are provided. Several model full core reactor problems (PWR, ABTR) are also presented along with openly accessible mesh files for reproducibility. A GUI for RGG tools (AssyGen and CoreGen) is available currently in the open-source domain for all popular operating systems and can significantly simplify the complexity in mesh generation for nuclear reactors.